POSITION OF MOON AND EARTH OFFERS RARE CHANCE TO PROBE MILKY WAY

By WALTER SULLIVAN

Published: February 17, 1987

MAUNA KEA, Hawaii—
An unusual alignment of the Earth, the moon and the core of the Milky Way galaxy is giving astronomers a rare opportunity to study that core, a mysterious region hidden from sight where processes not found elsewhere in the galaxy are at work.

Those processes generate radio and infrared radiation that traverses the intervening clouds of dust and gas. As the moon sweeps across the sources of this radiation, it cuts them off one by one from observers on Earth. Astronomers measuring when, where and how long the radiation is blocked can determine the size of its sources.

The observations from these occultations may help astronomers resolve such questions as whether the core of the galaxy is the scene of explosive events that generate new atoms and great shock waves of energy. Although the alignments occur every 18 years, this is the first series in which scientists have had the advanced equipment, including infrared scanners, they need to make useful observations.

For a limited period every 18 years, the Earth is aligned with the core of the galaxy in such a way that the moon passes across the core for a few seconds once a month. The moon's orbit carries it directly in front of the radiation sources, which occupy a tiny patch of sky in the constellation Sagittarius. Like a solar eclipse, each occultation can be observed from only a limited area on Earth.

One occultation was recorded from here on Jan. 26, and another will be studied here on April 18. The most revealing observation was made Sept. 11 from La Palma in the Canary Islands. Four objects were detected very close to an extremely powerful source of radio waves at or near the center of the galaxy. The objects were emitting powerful infrared radiation.

As the moon passed in front of three of them, their emissions disappeared abruptly, leading astronomers to conclude that the objects could not be more than a few million miles in diameter. But emissions from the fourth faded gradually, indicating that it was a broader source.

It was too broad to be starlike and too small to be a cluster of stars. What it is ''is anybody's guess,'' said Dr. Eric E. Becklin of the University of Hawaii, one of the observers. ''It's a very weird phenomenon.''

The University of Hawaii provides supporting services to all the observatories clustered atop this volcano, including the NASA Infrared Telescope Facility and the United Kingdom Infrared Telescope with which the occultations are being observed. The site, at an altitude of almost 14,000 feet, is ideal because much less infrared radiation from space is absorbed by the atmosphere.

Infrared and radio observations have already led to deeper knowledge of the galaxy's core, which for many astronomers is the most intriguing region of the sky. Objects in the center of the Milky Way do not occur elsewhere in the galaxy, but they appear to occur in the cores of most, if not all, other galaxies. A greater understanding of them could help explain how and why galaxies themselves are formed.

In particular, such understanding could account for the extraordinary amounts of energy emitted from galaxy cores. Such highly active cores radiate plasma jets in opposite directions at close to the speed of light.

Patterns of energy production in the Milky Way's core imply that the energy may be generated by a ''black hole,'' matter so dense and massive that its gravity prevents even the escape of light.

Because the core of the Milky Way is relatively close, it may ''serve as the Rosetta stone'' for deciphering galactic nuclei, according to Professor K. Y. Lo of the University of Illinois, who summarized recent discoveries regarding the Milky Way core in the journal Nature.

Enough has already been learned to begin imagining what a trip to the core might be like. The galaxy is a flattened, spiral assemblage of billions of stars. One, the Sun, lies in the outer part of the galaxy, 30,000 light years from the center; one light year is the distance light travels in a year at 186,000 miles a second. Only nearby parts of the galaxy are visible as the Milky Way. From observation of other spiral galaxies, however, it is assumed that the central region from which the Milky Way's spiral arms originate is a brilliantly glowing bulge dense with stars.

Observations of infrared radiation from the core indicate there may be 100 million times more stars there, per cubic light year, than in the outer parts of the galaxy. A voyager to the heart of the galaxy would find the sky almost blindingly bright with stars.

According to recent observations by the Very Large Array of radio telescopes in New Mexico of the area within 200 light years of the Milky Way's center, the traveler would enter a region of gigantic, parallel gaseous filaments, arcing around the core, each about 100 light years long. Additional clouds reaching from there toward the core are cut by strange, narrow ''threads.''

Scientists suspect that this material is in violent motion, but whether it is falling into the core, circling it or being ejected has not yet been determined.